Note: Descriptions are shown in the official language in which they were submitted.
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CORROSION AND ABRASION RESISTANT ALLOY
This invention relates to a cast alloy having combined
corrosion and abrasion resistance.
Applicant is aware of the following U.S. patents, the
disclosures of which may be referred to for background
material to the invention: 2,212,496; 2,311,878; 2,323,120;
3,165,400; 3,250,612; 3,876,475 and 3,941,589 and United
Kingdom 362,975 of 1931.
Equipment used in corrosive environments is typically
constructed of stainless steel or other high alloy materials.
These alloys provide excellent service in clear fluids.
However, when subjected to a corrosive slurry, fluid
containing abrasive solids, under moderate to high velocity,
these materials perform poorly due to poor abrasion
resistance.
Equipment used in abrasive slurry environments is
typically constructed of wear resistant irons. Wear resistant
irons provide excellent service in neutral slurries. However,
if the slurry becomes mildly acidic, these materials fail in
short order due to inadequate corrosion resistance.
The alloy of this invention provides superior combined
corrosion and abrasion resistance for handling acidic
slurrles .
An application requiring such a material is the
production of wet process phosphoric acid. The initial step
in the process is the reaction of raw phosphate ore with
concentrated sulphuric acid. Products of the reaction are
phosphoric acid and calcium sulphate, along with both chemical
and solid impurities. A typical product slurry analysis is
42~ phosphoric acid, up to 1~ chlorine and fluorine
impurities, approximately 2.5~ sulphuric acid and 30 to 40~
solids. The solids are mostly calcium sulphate and siliceous
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gangue (which is highly abrasive). The operating temperature
for raw acid formation and the slurry temperature, is usually
above 50C, typically 80C. The alloy of the invention can be
expected to offer significantly improved life compared to
either stainless steels or wear resistant irons for fluid
handling equipment and filtration equipment in this
environment.
The advantages of applicant's invention are achieved by
a cast, high chromium, ferritic, white iron alloy possessing
combined corrosion and abrasion resistance in both the as-cast
and age hardened condition.
The invention in one aspect pertains to a white iron
alloy wherein the improvement comprises a high chromium iron
base having a ferritic matrix containing a dispersed phase,
the alloy containing between about 26 to 28~ chromium, between
about 0.9 to l.2~ carbon, between about 0.4 to 0.75% silicon
and between about 0.5 to 1.0~ tungsten and a portion of the
tungsten being present in the dispersed phase, the alloy
having substantial resistance to combined corrosion and
abrasion in hot acid slurries.
The invention further pertains to a white iron alloy
having a high chromium iron base, the alloy having a ferritic
matrix containing a dispersed phase, the dispersed phase being
about 20 to 40~ of the total alloy and containing dispersed
high alloy carbides. The alloy contains between about 24 to
30~ chromium, between about O.S to 1.0~ tungsten, between
about 2.0 to 3.0~ molybdenum, between about 2.0 to 2.5
manganese, between about 1.0 to 2.0~ copper, between about
0.75 to 1.5~ carbon and up to about 0.85~ silicon.
Typically the alloy contains from between about 0.75~ to
1.5~ carbon, up to about 0.85 silicon, between about 2.0~ to
2.5~ manganese, between about 2.0~ to 3.0~ molybdenum, between
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i .
about 1.0~ to 2.0~ copper, between about 0.5~ to 1.0~
tungsten, between about 24~ to 30% chromium and the balance
being iron along with normal residual elements. Preferably,
the alloy contains between about 0.9 to 1.2~ carbon, between
about 26 to 28~ chromium and between about 0.4 to 0.75~
silicon. The silicon content should be kept as low as
possible, without reducing the castability of the alloy.
Silicon adds fluidity to the alloy melt. However, silicon can
reduce the corrosion resistance of the alloy in acidic media,
particularly in media containing halide ions. It is preferred
that the silicon level be as low as possible while maintaining
good castability in the alloy melt.
The combination of the alloying elements in the specified
proportions yields a material having an as-cast microstructure
of a high chromium ferritic matrix with approximately 30~ of
the alloy being a discontinuous complex phase. The
discontinuous phase contains high alloy chromium, molybdenum
and tungsten carbides which provide extreme hardness and
abrasion resistance to the alloy. The abrasion resistance can
be further enhanced, with little or no loss in corrosion
resistance, by a low temperature age hardening heat treatment.
The alloy in either the as-cast or age-hardened condition
possesses excellent combined corrosion and abrasion
resistance. The alloy is readily castable by standard foundry
practice and has adequate strength and ductility suitable for
mechanical rotating equipment.
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. ,
It is thus an object of applicant' 9 invention to provide
an alloy for use in acid slurries.
It is an object of applicant's invention to provide an
alloy which i8 resistant to the environments common in the wet
process production of phosphoric acid.
It is an object of applicant's invention to provide an
alloy which is resistant to abrasive conditions as found in
hot slurries.
It is an object of applicant's invention to provide an
alloy which has combined abrasion and corrosion resistance.
It is a further object of applicant's invention to
provide a white iron alloy which has mixed abrasion and
corrosion resistance.
It is an object of applicant's invention to produce a
white iron alloy having a ferritic matrix.
It is a further object of applicant's invention to
provide a white iron alloy having a dispersed phase in a
ferritic matrix, the dispersed phase containing carbides of
chromium, tungsten and molybdenum and producing an alloy
having high resistance to combined corrosive and abrasive
conditions.
It is a further object of applicant's invention to
provide a white iron alloy having corrosion resistance and
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abrasion resistance which is castable and hardenable.
The alloy of the invention is a high chromium white cast
iron. The alloy contains between about 0.75% to 1.5~ carbon,
between about 2.0~ to 2.5~ manganese, up to about 0.85%
silicon, between about 24% to 30% chromium, between about 2.0%
to 3.0% molybdenum, between about 1.0% to 2.0% copper, between
about 0.5% to 1.0% tungsten and the balance iron with minor
amounts of typical residual elements, such as sulphur and
phosphorous. It will be appreciated that the amount of
residues, such as sulphur, phosphorous and like materials is
kept below t,he level at which they would have a deleterious
effect on the properties of the alloy. Preferably the
aggregate of all such trace materials is below about 0.2~.
The principal alloying element of the white cast iron
alloy, after iron, is chromium which is typically present at
between about 24~ to 28% by weight, preferably 26~ to 28~. A
portion, typically 6 - 8~, based on the total alloy weight, of
the chromium is present as complex, extremely hard chromium
carbides, approximately 1400 Vickers hardness, providing
abrasion resistance. The balance of the chromium is present
in the matrix in solid solution, at a relatively high level of
approximately 20%, based on the total alloy weight, which
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1 337 1 60
provides corrosion resistance in oxidizing environments.
Carbon content is maintained at a level of between about
0.75~ to 1.5~. It is preferred that the carbon content be
between about 0.9 to 1.2~ and preferably toward the low end of
this range. Too high a carbon level results in the presence
of a dual phase matrix, the second phase being pearlite or
austenite, which can be subsequently transformed to
martensite, all of which exhibit poor corrosion resistance.
Carbon contents below about 0.75 to 0.9~ promotes a continuous
carbide network which impairs ductility.
The molybdenum content is maintained at a level of
between about 2.0~ to 3.0~. Molybdenum is a strong carbide
former and reacts with carbon preferentially to chromium, thus
freeing greater amounts of chromium for the matrix.
Molybdenum carbides are extremely hard, approximately 1500
Vickers hardness and improve the abrasion resistance. A
portion of the molybdenum content, between about 1.8 and 2.7~,
based on the total alloy weight, is found in the matrix,
between about 0.2 to 0.3~ by weight, based on the total alloy
weight, is present in the dispersed phase. The presence of
molybdenum in the matrix greatly enhances the general
corrosion resistance and provides resistance to pitting
corrosion in environments containing halide impurities.
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A copper content of between about 1.0~ to 1.5~, based on
the total weight of the alloy, i9 found in the matrlx. The
remaining copper is found in the dispersed phase. Copper is
known to improve the corrosion resistance in oxidizing
environments, such as phosphoric and sulphuric acid.
Tungsten addition of between about 0.5~ to l.0~ promotes
the formation of hard tungsten carbide, approximately 2400
Vickers hardness, which greatly improves abrasion resistance.
Tungsten forms carbide in preference to chromium, releasing
additional chromium to the matrix and thus, improving the
corrosion resistance. A portion of the tungsten content,
between about 0.4 to 0.~ of the total alloy, is found in the
matrix. Between about 0.1 to 0.2~ of the tungsten, based on
the total alloy, is found in the dispersed phase. The
tungsten may also be involved in the precipitation hardening
reaction.
The remainder of the alloy consists of iron and residual
elements and impurities, such as phosphorous and sulphur.
As-cast alloy exhibits a two phase structure having a
ferritic matrix and a discontinuous phase containing high
alloy metal carbides, primarily chromium, molybdenum and
tungsten carbides. The discontinuous phase is between about
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20 to 40~ of the total alloy, preferably about 30~. The
as-cast alloy exhibits excellent combined corrosion abrasion
resistance in applications such as pumping of slurries of
acidified phosphate ore. The alloy may also be suitable for
S service where resistance to galling is of importance.
The alloy may be hardened with a low temperature
precipitation hardening heat treatment, for example at about
2 to 4 hours at about 600 to 1800F. Applicant's material
shown in Tables II and III was hardened at about 900F for
about six hours. The hardened alloy provides improved
abrasion resistance with little or no loss in corrosion
resistance. Hardness varies from 30 to 40 Rockwell C.
The following tables show examples of alloys made within
the concepts of the invention compared with conventional
alloys. CF8M and CD4MCu alloys are commercially available
cast stainless steel alloys. The 15Cr-3Mo iron is a
commercially available cast abrasion resistant iron; it was
quenched and tempered to 65 Rockwell C hardness.
Experimental material shown in Table IA was made in a
conventional electric furnace by melting the ingredients
together in the proper proportions, deoxidizing and casting
test material using conventional gravity casting techniques.
The cast material was subjected to the tests shown in Tables
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II and III.
Table II summarizes the comparison of corrosion testing
of these alloys in the environment noted in Table II. The
alloys were prepared as conventional test blanks and subjected
to a serles of corrosion tests. A series was tested in
phosphoric acid at 90C. The test was run for 96 hours. The
phosphoric acid was a crude phosphoric acid typical of acids
used in producing phosphate fertilizer using Florida phosphate
rock. The acid contained approximately 1.25 percent fluoride
ion in 42 percent H3PO4. This acid composition is typical of
those which would be encountered in phosphoric acid
environment 9 .
As can be seen from Table II, applicant~s new alloy in
particular tested as being comparable to conventional cast
materials in static tests. The 42~ H3PO4 solutions are typical
of environments encountered in phosphoric acid production.
In Table III a number of alloys were subjected to the
combined effects of corrosion and abrasion. Tes~ing was done
in a laboratory test stand. Test samples were cast four blade
propellers with a diameter of approximately 9 inches. Each
propeller was rotated in an acidic slurry at 578 RPM, which
resulted in a tip speed of 22.7 Ft/Sec. Slurry analysis was:
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20~ by weight sollds (SiO2) 2.5 ~ sulphuric acid (pH = O).
Testing temperature was 50C. Test duration was 24 hours. As
-- can be seen, the alloy exhibits greatly superior resistance to
corrosion and abrasion in acidic slurries.
Evaluation of the castability of the experimental alloys
was made by making experimental castings of the general type
used in this service. These included pump casings. The
molten metal exhibited adequate fluidity filling all voids in
the molds.
Various changes and modifications may be made within the
purview of this invention, as will be readily apparent to
those skilled in the art. Such changes and modifications are
within the scope and teachings of this invention as defined by
the claims appended hereto. The invention is not to be
limited by the examples given herein for purposes of
illustration, but only the scope of the appended claims and
their equivalents.
TA~3LE lA
Summary-Experimental Heats
Analysis Weight Percent
Element N3596S525 S644 N6977N7038R0172
Carbon 1.451.04 1.29 1.09 1.14 .97
Mn 2.402.38 2.52 2.21 2.19 2.34
P .008.020 .021 .014 .016 .020
S .012.017 .017 .017 .016 .018
Si .85 .68 .70 .73 .74 .78
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Cr27~9627.71 26.30 27.3926~5327.15
Ni .16.20 .23 .19 .20 .27
Mo2.033.00 2.50 2.68 2.50 2.78
Cu1.271.23 1.01 .99 1.06 1.22
W .60.62 ~69 .66 .80 .65
Fe BalBal Bal Bal Bal Bal
TABLE IB
AnalysiS of Other Alloys Tested - Weight Present
Element CD4MC~ CF8M15Cr-3Mo Iron
C .21 .06 2.78
Mn .78 .70 ~.59
P .032 NA .011
S .013 NA .049
Si .59 1.57 .55
Cr27.6718.72 15.81
Ni 8-05 9~26 --
Mo2.19 2.29 1.80
Cu3.37 .55 --
Fe Bal Bal Bal
TABLE II
Static Corrosion Laboratory Tests in
42% H3PO4 and 98% H2SO4
Rates-mils per year (0.001 inch per year)
Materlal Heat Treatment H3PO4H2S4
N3695 As Cast 3.2 4.2
N3596 Hardened 3.5 --
S525 As Cast 4~5 12 7
S525 Hardened 1.0 --
N6977 As Cast 0.6
N6977 Hardened 2.0 --
N7038 AS Cast 1.5
N7038 Hardened 4.4 --
CF8M Soln Annealed 0.2 20.0
ASTM-A743,
Grade CF8M
CD4MCu Soln Annealed 1.0 1.7
ASTM-A743,
Grade CD4MCu
. . = . .
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TABLE III
Dynamlc Corroslon Abraslon Tests
Rates-mils per year (0.001 lnch per year)
Materlal Heat Treatment Rate
N6977 As Cast 160
Hardened 92
N7038 As Cast 110
Hardened 94
R0172 As Cast 131
Hardened 101
S525 As Cast 86
E~ardened 83
S644 As Cast 166
Hardened 137
CE~8M Soln Anneal,250
ASTM-A743,
Grade CF8M
CD4MCu Soln Anneal,209
ASTM-A743,
Grade CD4MCu
15Cr-3Mo
Wear Reslstant Hardened, 12,037
Iron quenched and
tempered
ASTM-A532, Class
II, type C
